The present invention relates to the mapping of an underground cable or pipe. It also relate to an interrupter for interrupting the cathodic protection voltage applied to such an underground cable or pipe.
It is well known to apply a cathodic (negative) voltage to an underground cable or pipe (hereinafter “pipe”) to reduce corrosion of that pipe. If the pipe is at a positive voltage relative to ground, electrolytic effects occur which damage the pipe. It should be noted that such cathodic protection voltage may be applied even when the pipe is coated to insulate it from the ground, because it is common for that insulation to develop holes or other faults which could result in localised damage.
However, it is also common for such pipes to experience electrical fields due to other objects such as adjacent pipes, or other electrical conductors such as railway lines, etc. Such localised fields sometimes result in the pipe experiencing a positive voltage relative to ground, so that corrosion occurs. It is therefore desirable to investigate currents in the pipe or cable to detect points where corrosion may occur. If stray currents are detected, these may be due to electric fields from other objects (such currents hereinafter being referred to as stray currents), appropriate action can be taken, such as repairing the fault or taking corrective action elsewhere by suitable voltage control, or even by providing sacrificial anodes at an adjacent fault.
In order to detect the stray currents, it is possible to make use of the magnetic fields generated by those currents, and detect those fields remotely from the pipe itself, such as at the surface. Detection of such magnetic fields is generally known, although special techniques may be needed because of the low magnitude of the currents.
There is also the problem that there may be several pipes in the area being investigated, and the currents from those other pipes may confuse the measurement.